Metered dose syringe

Abstract

A metered dose syringe 1 comprising a barrel 2 enclosing an internal volume (3, figure 3C) for holding liquid, a plunger 4, and a sleeve 5 received over and helically coupled to the barrel. The coupling includes at most one threaded surface 6, and the sleeve is configured to restrict travel of the plunger when depressed such that rotating the sleeve relative to the barrel control a maximum delivery volume. The pitch of the helical coupling may vary along a range of motion, and include a guide element arranged to run along the helical track. Indicia may be provided to indicate a dose or a volume associated with relative sleeve positions, the sleeve having a window to provide visibility thereof. The coupling may generate audible and / or haptic indication in response to a position of the sleeve corresponding to a pre-set position. A locking mechanism may secure the position of the sleeve relative to the barrel.

Description

Field The present invention relates to metered dose syringes, pre-filled metered dose syringes and methods for using the same. The present invention relates in particular to metered dose syringes for use in providing medicaments in time-pressured situations. Background Medication commonly used in emergency or similarly time-critical situations may often need to be drawn up into a syringe or similar device from a container. Accidental drawing up of an incorrect volume and / or of the wrong dilution of medicament can lead to negative outcomes from under- or over-dosing the patient. Examples of medicaments commonly used in time-pressured situations may include adrenaline for cardiac arrest, amiodarone for shock-refractory cardiac arrest, lorazepam for seizures, glucose for hypoglycaemia and so forth. The correct dose of a given medicament will depend on the weight of that patient. Whilst pre-filled syringes of fixed volume may be useable for a majority of adults of healthy weight, it would still be preferable for doses to be more specific to patients (depending on height and other factors, adult weights may still vary from as low as 45 kg to 100 kg or more without being overweight or obese). The problem is significantly more pronounced in paediatric medicine. In addition to the commonly encountered problem of dosing errors, the process of drawing up medicaments into a syringe is time consuming, and speed is essential in many of the relevant clinical scenarios. US 2021 / 213208 Al describes a device which allows a limit to be set for a volume which may be drawn up into a syringe. US 9,302,052 Bl describes methods and devices for metering and delivering a desired dose of medication from a syringe. Summary According to a first aspect of the invention, there is provided a metered dose syringe including a barrel enclosing an internal volume for holding liquid. The metered dose syringe also includes a plunger. The metered dose syringe also includes a sleeve received over and helically coupled to the barrel. The helical coupling between the sleeve and barrel includes at most one threaded surface. The sleeve is configured to restrict travel of the plunger when depressed, such that rotating the sleeve relative to the barrel controls a maximum delivery volume from depressing the plunger. The barrel has an outlet for discharging liquid stored in the internal volume. A filled capacity of the metered dose syringe may correspond to the portion of the internal volume between an outlet of the barrel and a head of the plunger. Rotation of the sleeve relative to the barrel via the helical coupling may not change the filled capacity. Instead, rotation of the sleeve relative to the barrel via the helical coupling simply controls a distance which the plunger may be depressed to eject liquid from the internal volume of the barrel. Unless clearly indicated to the contrary, references herein to "axial", "rotational" and "radial" movements and / or directions are in relation to cylindrical polar coordinates having an axis aligned with an axis of the barrel. Rotation of the sleeve relative to the barrel via the helical coupling may convert rotation of the sleeve relative to the barrel into axial translation, and vice versa. The ratio between axial and rotational movements may be controlled by a pitch of the helical coupling (i.e. the amount of axial translation per angle of rotation). No part of the sleeve may contact the interior volume. No part of the sleeve may be received within the internal volume. A pitch of the helical coupling may vary along a range of motion of the helical coupling. In other words, as the sleeve is rotated relative to the barrel, the corresponding rate of axial translation may change as a function of the position of the sleeve relative to the barrel (within the range permitted by the helical coupling). In this way, the sensitivity of the rotation of the sleeve to set a dose may be varied across the range of motion of the sleeve relative to the barrel. This permits setting accurate doses for patients spanning a wide range of weights. This may be particularly useful in paediatric settings, allowing a low pitch at one end of the helical coupling range of motion for fine control of doses for newborns / infants, transitioning to a larger pitch as the sleeve continues to be rotated, so that doses may also be selected for older / larger infants, toddlers, and so forth. The position of the sleeve relative to the barrel may be measured in terms of a total axial displacement of the sleeve relative to the barrel compared to a starting or reference position. Position of the sleeve relative to the barrel may be measured in terms of a total displacement, relative to the starting or reference position, along a path corresponding to the range of motion of the helical coupling. In either case, the starting or reference position may correspond to one extreme of the range of motion of the sleeve relative to the barrel. Preferably, the starting or reference position corresponds to the end of the range of motion at which the delivery volume obtainable from depressing the plunger is as small as possible. For example, a small volume useable to prime the syringe by depressing the plunger to abut the sleeve, ejecting any gas (held with the barrel outlet uppermost in order to ensure any excess gas is ejected) and permitting accurate and reproducible zeroing of the range provided by rotation of the sleeve away from the starting position. The helical coupling may be configured with variable pitch in the form of two or more sections of the range of motion, each section having a different constant pitch. In other words, the pitch may include a step or abrupt transition between each section of the range of motion. The pitch may be a linear function of the position within the range of motion (for example axial or path length displacement). The pitch may be a reciprocal function of the position within the range of motion (for example axial or path length displacement). The pitch may vary with one of a square root, a reciprocal, a power or a logarithm of the position within the range of motion (for example axial or path length displacement). The pitch may vary within the range of motion (for example axial or path length displacement) in any manner desired, and is not limited to variations describable using closed-form mathematical functions. The helical coupling may include a helical track and a guide element arranged to run along the helical track. The range of motion of the helical coupling may correspond to the length of the helical track through which the guide element is able to run. The helical track may also be described and / or referred to as a helical "groove" or "path". A portion of the guide element received into the helical track may be hemispherical. A portion of the guide element received into the helical track may be cylindrical. A portion of the guide element received into the helical track may be pyramidal. A portion of the guide element received into the helical track may be conical. A portion of the guide element received Into the helical track may be shaped like ratchet or gear tooth. A portion of the guide element received into the helical track may have any shape complementary to a profile of the helical track perpendicular to the helical track. The helical track may be straight-walled. Alternatively, one or both walls defining the helical track may be tapered. The walls defining the helical track may have a profile along the length of the helical track which is a smooth curve. Alternatively, the walls defining the helical track may have a profile along the length of the track which is saw-toothed, geared, ratcheted, stepped, and so forth. The helical track may be formed in the barrel. The guide element may be a first element coupled to, or integrally formed with, the sleeve. The first element may not be a thread. The first element may take the form of an extension or protrusion projecting radially inwards from the sleeve. The first element may be coupled to the sleeve via one or more bearings arranged to permit movement of the first element, or a portion of the first element which contacts the helical track, radially relative to the sleeve. The one or more bearings may minimise or constrain axial and / or rotational movements of the first element relative to the sleeve. In other words, the first element and sleeve may be constrained to move together axially and rotationally, relative to the barrel. The one or more bearings may include, without being limited to, one or more of plain bearings, ball bearings, springs, flexures, flexures integrally formed with the sleeve, or any other bearing type(s) known to be suitable to provide the relative motions and constraints described hereinbefore. The helical track may be formed In the sleeve. The guide element may be a second element coupled to or integrally formed with the barrel. The second element may not be a thread. The second element may take the form of an extension or protrusion projecting radially outwards from the barrel. The second element may be coupled to the barrel via one or more bearings arranged to permit movement of the second element, or a portion of the second element which contacts the helical track, radially relative to the barrel. The one or more bearings may minimise or constrain axial and / or rotational movements of the second element relative to the barrel. In other words, the second element and barrel may be constrained to move together axially and rotationally, relative to the sleeve. The one or more bearings may include, without being limited to, one or more of plain bearings, ball bearings, springs, flexures, flexures integrally formed with the barrel, or any other bearing type(s) known to be suitable to provide the relative motions and constraints described hereinbefore. One or more indicia may be provided on the barrel and / or on the sleeve. The one or more indicia may indicate a dose or volume associated with the relative positions of the sleeve and barrel. The indicia may indicate a volume which is dispensable by depressing the plunger until it abuts the sleeve. When the syringe is for use with a particular medicament at a particular concentration, the indicia may indicate a dose of the particular medicament which is dispensable by depressing the plunger until it abuts the sleeve. Alternatively, when the syringe is for use with a particular medicament at a particular concentration, the indicia may indicate a weight or a weight range for whom the corresponding volume represents the correct dose. Alternatively, when the syringe is for use in paediatric medicine with a particular medicament at a particular concentration, the indicia may indicate an age of the patient. The age of the patent may correspond to a dose suitable for the average weight of a patient having the indicated age. The sleeve may include a window arranged to provide visibility of indicia provided on the barrel. The window may be sized to allow only a single indicia provided on the barrel to be visible at once. This may help to prevent confusion between adjacent and / or closely spaced indicia. The helical coupling may be configured to generate an audible and / or haptic indication in response to a position of the sleeve relative to the barrel corresponding to one of a number of pre-set relative positions. Each pre-set relative position may correspond to one of the indicia. When the syringe is for use with a particular medicament at a particular concentration, each pre-set position may correspond to a particular dose. For example, each pre-set position may correspond to a dose corresponding to a patient age, weight, weight range and so forth. Each of the number of pre-set relative positions are provided by corresponding recesses formed in the helical track. Each recess may have increased depth and / or width compared to intervening section of the helical track. Transitions between each recess and intervening sections of helical track may be smooth. Transitions between each recess and intervening sections of helical track may be sharp, for example a step edge. Transitions between each recess and intervening sections of helical track may be smooth on one side and sharp on the other side, for example so as to facilitate latching of the guide element into the recess. The first / second element may be urged towards the helical track by biasing means. Biasing means may take the form of a spring, a flexure, an elastomeric member, and so forth. In this way, the first / second element will be pushed into the recess. Further movement will require moving the element out of the recess against the action of the biasing means, which will provide haptic feedback to a user. Each recess may take the form of a detent. In other words, the sleeve and barrel may "lock in" to a pre-set position until force exceeding a threshold is applied and / or a release catch is actuated. If the edge of the recess (or detent) is configured with a sharp transition, an appropriately shaped guide element (for example sharp on a trailing side) may generate an audible "click" as the guide element passes the sharp transition and is accelerated into the recess by the biasing means. Alternative mechanisms for generating an audible and / or haptic "click" may be employed. Alternatively, instead of recesses in the helical track, each of the number of pre-set relative positions may be provided by corresponding protrusions extending into the helical track from a floor (or base) and / or walls of the helical track. The metered dose syringe may also include a locking mechanism actuatable to secure the relative position of the sleeve relative to the barrel. The locking mechanism may include, or take the form of, a screw. When the helical coupling takes the form of a helical track and a guide element, the screw may be arranged to Increase a force biasing the guide element towards the helical track when tightened. Alternatively, the locking mechanism may include, or take the form of, a clutch. The locking mechanism may include, or take the form of, a wedge. The locking mechanism may take the form of a spline. The locking mechanism may take the form of a spring ratchet mechanism, for example cooperating with the one or more recesses or detents. The locking mechanism may include, or take the form of, an irreversible snap-fit mechanism. The locking mechanism may include, or take the form of, a friction fit mechanism. The locking mechanism may include, or take the form of, a cantilever arm. In other words, the locking mechanism may take any form previously employed in the field of mechanics for the purposes of resisting and / or preventing relative motion between a pair of parts. The locking mechanism may be actuatable at any point within the range of motion of the helical coupling. The locking mechanism may be additionally secure when actuated at one of the plurality of pre-set relative positions. Alternatively the locking mechanism may only be actuatable when coincident with a pre-set relative position. Howsoever provided, the locking mechanism should provide sufficient constraint to prevent accidental movement of the sleeve relative to the barrel during depression of the plunger to deliver the desired and / or set a dose or volume, and / or when twisting or pressing to connect the outlet of the barrel to a needle, a cannula, and so forth. Additional security / safety may be provided by combining the locking mechanism with the plurality of pre-set relative positions. The metered dose syringe may also include a locking mechanism configured to engage at each of the plurality of pre-set relative positions, so as to secure the relative position of the sleeve relative to the barrel. The locking mechanism may include a release mechanism actuatable to release the locking mechanism and permit further relative movement of the sleeve and barrel. An outlet of the barrel may be configured for coupling via a Luer taper, a Luer slip, a Luer lock, a threaded connection, or a snap-fit connection. The outlet of the barrel may be configured for coupling using any standard fitting such as, for example, an NRFit (RTM) connection. A removeable cap may be positioned to cover the outlet of the barrel. The removeable cap may be single use such that once first removed, the cap may not be replaced. This may provide visual evidence that a syringe has been damaged or tampered with, and may help to prevent improper re-use of the metered dose syringe. The outlet of the barrel may be coupled to an integral needle. The metered dose syringe may also include a capture mechanism configured to secure the plunger to the sleeve once the plunger has been depressed to deliver the maximum delivery volume set using the sleeve. The capture mechanism may take the form of a catch or similar mechanism configured to lock the plunger to the sleeve and prevent withdrawal of the plunger. The sleeve may prevent further depression of the plunger. The capture mechanism may also be configured to prevent relative rotation of the sleeve relative to the barrel once the plunger has been secured to the sleeve. In this way, after being first used as Intended, the metered dose syringe cannot be further used and / or re-used. For example, a one-directional ratchet mechanism may be coupled between the plunger and the barrel, or between the plunger and the sleeve. Alternatively, the plunger and sleeve may be configured such that, when the plunger abuts the sleeve to prevent further depression, a top of the plunger will be surrounded by, and lush with, a top of the sleeve. In this was, it may not be possible to draw the plunger out again without additional tools. The capture mechanism may further include a visual indicator that the plunger has been secured to the sleeve. This may provide visual confirmation to a user that the used metered dose syringe has been rendered inoperable. The capture mechanism may require a user to engage, but once engaged may not be reversible without physically breaking the capture mechanism. The capture mechanism may engage automatically when the plunger Is depressed with sufficient force whilst abutting the sleeve. The force required to engage the capture mechanism may be configured in order to avoid accidental engagement when priming the metered dose syringe. The capture mechanism may be configured to provide an audible and / or haptic feedback when it is engaged, for example a "click". In this way, an authorised user can be certain both that they have delivered the entire dose they set by rotating the sleeve, and that they have made the syringe safe against misuse. A pre-filled syringe may include, or take the form of, the metered dose syringe, having the internal volume of the barrel filled with a liquid medicament. The medicament may include one or more active ingredients dissolved in the liquid. The medicament may include one or more active ingredients suspended in the liquid. The medicament may include one or more active ingredients emulsified in the liquid. The medicament may include, or take the form of, adrenaline. The medicament may include, or take the form of, adrenaline at a dilution of 1:10,000, which may alternatively be expressed as 100 micrograms per millilitre. In a case where the pre filled syringe is intended for paediatric use, a dose may be determined corresponding to 10 micrograms of adrenaline per kilogram of the patient weight. The pre-filled syringe may include at least pre-set positions corresponding to doses of 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 micrograms of adrenaline. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; • More than 110 • More than 130 • More than 150 • More than 170 • More than 190 • More than 215 • More than 245 • More than 280 • More than 340 • More than 440 • More than 750 micrograms and less micrograms and less micrograms and less micrograms and less micrograms and less micrograms and less micrograms and less micrograms and less micrograms and less micrograms and less micrograms. than or equal to 130 than or equal to 150 than or equal to 170 than or equal to 190 than or equal to 215 than or equal to 245 than or equal to 280 than or equal to 340 than or equal to 440 than or equal to 750 micrograms; micrograms; micrograms; micrograms; micrograms; micrograms; micrograms; micrograms; micrograms; micrograms; and The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; • More than 110 micrograms and less than or equal to 130 micrograms; • More than 130 micrograms and less than or equal to 150 micrograms; • More than 150 micrograms and less than or equal to 170 micrograms; • More than 170 micrograms and less than or equal to 190 micrograms; • More than 190 micrograms and less than or equal to 215 micrograms; • More than 215 micrograms and less than or equal to 245 micrograms; • More than 245 micrograms and less than or equal to 280 micrograms; • More than 280 micrograms and less than or equal to 340 micrograms; • More than 340 micrograms and less than or equal to 440 micrograms; and • More than 440 micrograms and less than or equal to 750 micrograms. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; • More than 110 micrograms and less • More than 130 micrograms and less • More than 150 micrograms and less • More than 170 micrograms and less • More than 190 micrograms and less • More than 215 micrograms and less • More than 245 micrograms and less • More than 280 micrograms and less • More than 340 micrograms and less than or equal to 130 micrograms; than or equal to 150 micrograms; than or equal to 170 micrograms; than or equal to 190 micrograms; than or equal to 215 micrograms; than or equal to 245 micrograms; than or equal to 280 micrograms; than or equal to 340 micrograms; and than or equal to 440 micrograms. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; • More than 110 micrograms and less than or equal to 130 micrograms; • More than 130 micrograms and less than or equal to 150 micrograms; • More than 150 micrograms and less than or equal to 170 micrograms; • More than 170 micrograms and less than or equal to 190 micrograms; • More than 190 micrograms and less than or equal to 215 micrograms; • More than 215 micrograms and less than or equal to 245 micrograms; • More than 245 micrograms and less than or equal to 280 micrograms; and • More than 280 micrograms and less than or equal to 340 micrograms. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; • More than 110 micrograms and less than or equal to 130 micrograms; • More than 130 micrograms and less than or equal to 150 micrograms; • More than 150 micrograms and less than or equal to 170 micrograms; • More than 170 micrograms and less than or equal to 190 micrograms; • More than 190 micrograms and less than or equal to 215 micrograms; • More than 215 micrograms and less than or equal to 245 micrograms; and • More than 245 micrograms and less than or equal to 280 micrograms. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: Less than or equal to 35 micrograms; More than 35 More than 45 More than 60 More than 85 micrograms and less than micrograms and less than micrograms and less than micrograms and less than or or or or equal to 45 micrograms; equal to 60 micrograms; equal to 85 micrograms; equal to 110 micrograms; • More than 110 micrograms and less than or equal to 130 micrograms; • More than 130 micrograms and less than or equal to 150 micrograms; • More than 150 micrograms and less than or equal to 170 micrograms; • More than 170 micrograms and less than or equal to 190 micrograms; • More than 190 micrograms and less than or equal to 215 micrograms; and • More than 215 micrograms and less than or equal to 245 micrograms. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; • More than 110 micrograms and less than or equal to 130 micrograms; • More than 130 micrograms and less than or equal to 150 micrograms; • More than 150 micrograms and less than or equal to 170 micrograms; • More than 170 micrograms and less than or equal to 190 micrograms; and • More than 190 micrograms and less than or equal to 215 micrograms. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; • More than 110 micrograms and less than or equal to 130 micrograms; • More than 130 micrograms and less than or equal to 150 micrograms; • More than 150 micrograms and less than or equal to 170 micrograms; and • More than 170 micrograms and less than or equal to 190 micrograms. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; • More than 110 micrograms and less than or equal to 130 micrograms; • More than 130 micrograms and less than or equal to 150 micrograms; and • More than 150 micrograms and less than or equal to 170 micrograms. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; • More than 110 micrograms and less than or equal to 130 micrograms; and • More than 130 micrograms and less than or equal to 150 micrograms. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; and • More than 110 micrograms and less than or equal to 130 micrograms. The pre-filled syringe may have sequential pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; and • More than 85 micrograms and less than or equal to 110 micrograms. The medicament may include, or take the form of amiodarone, calcium chloride, atropine, or any other substance known for use in treating or managing cardiac arrest and producible in liquid form (whether dissolved or in suspension). The medicament may include, or take the form of glucose, lorazepam, adenosine, or any other substance known for use in treating or managing medical emergencies / critical injuries and producible in liquid form (whether dissolved or in suspension). The medicament may include, or take the form of propofol, ketamine, rocuronium, fentanyl, alfentanil, midazolam, morphine, metaraminol, ephedrine, phenylephrine, or any other substance known for use in Anaesthesia and / or Rapid Sequence Induction. The pre-filled syringe may have the relative position of the sleeve and barrel set to a start position corresponding to an extreme end of the helical coupling range of motion which minimises the travel of the plunger when depressed. The internal volume may be filled such that the plunger may be depressed to discharge a pre-determined priming volume before abutting the sleeve in the start position. In this way, the pre-filled syringe may be used by first removing it from sterile packaging (for hygiene and infection control), then priming the syringe to ensure any gas is removed by depressing the plunger until it abuts the sleeve in the start position. During this priming step, the pre-filled syringe should be "Inverted", which is to say held with the outlet of the barrel uppermost. This may also help to ensure a reliable zero-level before setting the dispensable volume. Once the syringe in primed, the sleeve and barrel may be rotated (with axial translation generated by the helical coupling) to set the required volume / dose. If included, the locking mechanism may be engaged. The syringe may be used to dispense the set volume / dose. The pre-filled syringe may be packaged in sterile packaging. The pre-filled syringe may include a cap or cover which must be removed from the barrel outlet prior to use. The cap or cover may be irreversibly removable, in order to facilitate detection of tampering. According to a second aspect of the Invention, there is provided a method of setting a metered dose syringe or a pre-filled syringe according to the first aspect. The method may include rotating the sleeve relative to the barrel to set a maximum delivery volume from depressing the plunger. The method may include features corresponding to any features of the metered dose syringe or the pre-filled syringe of the first aspect. Definitions applicable to the metered dose syringe or the pre-filled syringe of the first aspect (or features thereof) may be equally applicable to the method of the second aspect (or features thereof). The method may also include, before rotating the sleeve relative to the barrel to set the maximum delivery volume, priming the syringe by depressing the plunger to abut the sleeve In a starting position. The pre-filled syringe may be "inverted", which is to say held with the outlet of the barrel uppermost, during the priming step. The metered dose syringe or pre-filled syringe may include a locking mechanism actuatable to secure the relative position of the sleeve relative to the barrel. The method may also include, after rotating the sleeve relative to the barrel to set the maximum delivery volume from depressing the plunger, actuating the locking mechanism. The metered dose syringe or pre-filled syringe may include a locking mechanism configured to engage at each of a number of pre-set relative positions of the helical coupling, so as to secure the relative position of the sleeve relative to the barrel. The locking mechanism of the metered dose syringe may also include a release mechanism actuatable to release the locking mechanism and permit further relative movement of the sleeve and barre. The method may also include actuating the release mechanism one or more times whilst rotating the sleeve relative to the barrel to set the maximum delivery volume from depressing the plunger. An outlet of the barrel of the metered dose syringe or pre-filled syringe may be configured for coupling via a Luer taper, a Luer slip, a Luer lock, a threaded connection, or a snap-fit connection. The method may also include connecting the outlet of the barrel to an inlet via the respective Luer taper, Luer slip, Luer lock, threaded connection, or snap-fit connection. The inlet may be an inlet of a detachable needle. The inlet may be a tube. The inlet may be a port. The inlet may be a valve. The metered dose syringe or pre-filled syringe may include a capture mechanism configured to secure the plunger to the sleeve once the plunger has been depressed to deliver the maximum delivery volume set using the sleeve. The method may also include pressing the plunger until the capture mechanism engages to secure the plunger to the sleeve. Brief Description of the Drawings Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view of a first metered dose syringe; Figure 2A is a schematic perspective view of the first metered dose syringe; Figure 2B is an expanded view of the region labelled "A" in Figure 2A; Figure 2C is an expanded view of the region labelled "B" in Figure 2A; Figure 3A is a schematic side view of the first metered dose syringe, rotated 90° axially relative to Figure 1; Figure 3B is a cross-section along the plane denoted by label "C" in Figure 3A; Figure 3C is a cut-away projection from the same plane as Figure 3B; Figure 3D is a cross-section along the plane denoted by label "D" in Figure 3A and Figure 3E is a cut-away projection from the same plane as Figure 3D; Figure 4A is an exploded schematic side view of the first metered dose syringe; Figure 4B is an exploded schematic side view of the first metered dose syringe 1, rotated 90° axially relative to Figure 4A; Figure 4C is an exploded schematic perspective view of the first metered dose syringe; Figure 4D is an expanded view of the region labelled "E" in Figure 4B; Figure 4E is an expanded view of the region labelled "F" in Figure 4C; Figure 4F is a cross-section along the plane denoted by label "G" in Figure 4A; Figure 5A is a schematic projection view of a second metered dose syringe; Figure 5B shows an expanded view of the region labelled "H" in Figure 5A; Figure 6A is a schematic side view of the second metered dose syringe; Figure 6B is a cross-section along the plane denoted by label "K" in Figure 6A; Figure 6C is a cut-away projection from the same plane as Figure 6B; Figure 6D is a cross-section along the plane denoted by label "J" in Figure 6A; Figure 6E shows an expanded view of the region labelled "M" in Figure 6C; Figures 7A and 7B are photographs of a test metered dose syringe; Figure 8 presents comparative data for tasks completed using a conventional adrenaline bolus and using the test metered dose syringe; Figure 9A is a schematic sectional view of a first example configuration of pre-set positions of a helical track; Figure 9B is a schematic sectional view of a second example configuration of pre-set positions of a helical track; Figure 10A is a schematic sectional view of a third example configuration of pre-set positions of a helical track; Figure 10B is a schematic sectional view of a fourth example configuration of pre-set positions of a helical track; and Figure 11 reproduces a paediatric emergency drug chart published by the Resuscitation Council UK. Detailed Description of Certain Embodiments In the following description, like elements are denoted by like reference numerals. The present specification describes examples of a metered dose syringe which can be prefilled with a medicament commonly used in emergencies / time-pressured clinical situations. The medicament may include, without being limited to, adrenaline for cardiac arrest, amiodarone for shock-refractory cardiac arrest, lorazepam for seizures, glucose for hypoglycaemia, and so forth. The metered dose syringes in accordance with the present disclosure allow for quick, accurate and secure setting of a maximum delivery volume from depressing the plunger of the metered dose syringe. Experimental data obtained from trials with clinicians (see Table 1 and the corresponding description) demonstrates that metered dose syringes in accordance with the present disclosure both reduce dosing errors and also reduce the time to dose administration (see Figures 7A to 8). The metered dose syringes are preferably single use, and may be filled in advance of intended use, or more preferably may be pre-filled with the intended medicament and packaged in sterile packaging ready for use by medical personnel. The metered dose syringes in accordance with the present disclosure may be produced to interface with any commonly used ports / tubes / needles such as, for example, the Luer lock or Luer slip of a standard peripheral intravenous cannula, intraosseous access device, or central vascular access device. In other examples of metered dose syringes in accordance with the present disclosure, the metered dose syringe may be produced with an integrated needle covered by a removable cap until the time of use. First metered dose syringe Referring to Figure 1, a first example of a metered dose syringe 1 (hereinafter the "first metered dose syringe") is shown in a schematic side view. Referring also to Figures 2A to 2C, Figure 2A shows the first metered dose syringe 1 in a schematic perspective view. Figure 2B shows an expanded view of the region labelled "A" in Figure 2A. Figure 2C shows an expanded view of the region labelled "B" in Figure 2A. Referring also to Figures 3A to 3E, Figure 3A shows the first metered dose syringe 1 In a schematic side view rotated 90° axially relative to Figure 1. Figure 3B shows a cross-section along the plane denoted by label "C" in Figure 3A and Figure 3C shows a cut-away projection from the same plane. Figure 3D shows a cross-section along the plane denoted by label "D" in Figure 3A and Figure 3E shows a cut-away projection from the same plane. Referring also to Figures 4A to 4F, Figure 4A shows an exploded schematic side view of the first metered dose syringe 1 from the same direction as Figure 1. Figure 4B shows an exploded schematic side view of the first metered dose syringe 1 from the same direction as Figure 3A. Figure 4C shows an exploded schematic perspective view of the first metered dose syringe 1. Figure 4D shows an expanded view of the region labelled "E" In Figure 4B. Figure 4E shows an expanded view of the region labelled "F" in Figure 4C. Figure 4F shows a cross-section along the plane denoted by label "G" in Figure 4A, with the outline of the barrel 2 when received indicated using dashed lines. The first metered dose syringe 1 Includes a barrel 2 enclosing an internal volume 3 for holding liquid (not shown), a plunger 4, and a sleeve 5 received over and helically coupled to the barrel 2. The helical coupling between the sleeve 5 and the barrel 2 includes at most one surface which might be threaded (or considered so due to presence of a helical groove or protrusion), which in the case of the first metered dose syringe 1 is the outer cylindrical surface of the barrel 2, in which a helical groove 6 is formed. The role of the helical groove 6 in the helical coupling between the sleeve 5 and the barrel 2 is described in further detail hereinafter. As will be further described with reference to Figures 1 to 4F, the sleeve 5 is configured to restrict travel of the plunger 4 when depressed, such that rotating the sleeve 5 relative to the barrel 2 controls a maximum delivery volume of liquid stored in the internal volume 3 from depressing the plunger 4. In The first metered dose syringe 1, no part of the sleeve 5 contacts the internal volume 3, and no part of the sleeve 5 is received within the internal volume 3. This may allow using lower grade materials for the sleeve without concern about chemical interactions with / contamination of a medicament held In the internal volume 3 during storage prior to use. Unless specified to the contrary, references herein to "axial", "rotational" and "radial" movements and / or directions herein are relative to cylindrical coordinates having an axis aligned with an axis of the cylindrical portion of the barrel 2. The barrel 2 includes an outlet 7 for discharging liquid stored in the internal volume 3. In the first metered dose syringe 1 the outlet 7 takes the form of a Luer taper (though the precise form is not essential). A filled capacity of the metered dose syringe 1 corresponds to the portion of the internal volume 3 between the outlet 7 of the barrel 2 and a head 8 of the plunger 4. The plunger head 8 forms a liquid-tight seal to the interior walls of the barrel 2 once Inserted. The plunger head 8 is coupled to a protrusion 9 formed at one end of a shaft 10 of the plunger 4, whilst the actuatable end 11 of the plunger 4 is formed at the opposite end of the plunger shaft 10. Liquid stored in the internal volume 3 is discharged from the outlet 7 by applying a force urging the actuatable end 11 of the plunger 4 towards the barrel 2 until the actuatable end 11 abuts the sleeve 5 and can be depressed no further. In the first metered dose syringe 1, the actuatable end 11 has a larger diameter than an internal diameter of an opening at the end of the sleeve 5 which receives the shaft 10 (furthest from barrel 2). The sleeve 5 is provided with a flange 12 to facilitate gripping when depressing the actuatable end 11 of the plunger 4. As shown for the first metered dose syringe 1, the actuatable end 11 of the plunger 4 may also be shaped or contoured to facilitate gripping by a user. It is important that the helical coupling, provided in the case of the illustrated first metered dose syringe 1 by helical groove 6, does not experience unwanted relative movement between the barrel 2 and sleeve 5 whilst the plunger 4 is depressed. Friction along the permitted directions of the helical coupling may be sufficient In some other examples, although as explained hereinafter the first metered dose syringe 1 includes means for additionally securing the barrel 2 relative to the sleeve 5 for added security. Rotation of the sleeve 5 relative to the barrel 2 via the helical coupling does not change the filled capacity, i.e. the volume between the outlet 7 and plunger head 8. Instead, rotation of the sleeve 5 relative to the barrel 2 via the helical coupling only controls the maximum distance which the plunger 4 may be depressed to eject liquid from the internal volume 3 of the barrel 2 before further movement is prevented by the sleeve 5. A helical coupling converts rotation of the sleeve 5 relative to the barrel 2 into axial translation. The ratio between axial and rotational movements may be controlled by a pitch of the helical coupling (i.e. the amount of axial translation per angle of rotation). In the first metered dose syringe 1 the helical coupling is formed by the cooperation of the helical track 6 with a guide element 13 arranged to run along the helical track 6. The range of motion of the helical coupling in this case corresponds to the length of the helical track 6 through which the guide element 13 Is able to run. The helical track 6 may also be described and / or referred to herein as a helical "groove" or "path". In the first metered dose syringe 1 the helical track 6 is formed in the barrel 2, and the guide element 13 is coupled to the sleeve 5. The guide element 13 is not anything which could be regarded as a thread cooperating with the helical track 6. Instead, referring in particular to Figure 4F, in the first metered dose syringe 1 the guide element 13 takes the form of a threaded ball and spring plunger. The guide element 13 includes a ball bearing 14 received within an axial, cylindrical hole 15 formed in the guide element 13. The ball bearing 14 is retained within the cylindrical hole 15 by an annular lip 16, and is biased towards the annular lip 16 by a spring 17 received between the ball bearing 14 and the closed end of the cylindrical hole 15. The ball bearing 14 and annular lip 16 are dimensioned so that the ball bearing 14 extends past the annular lip 16. A thread 18 is formed on the outer cylindrical surface of the guide element 13. The guide element 13 is received into an internally threaded aperture 19 formed in a cylindrical protrusion 20 which extends radially outwards from the sleeve 5. Optionally, and as illustrated for the first metered dose syringe 1, the walls defining the cylindrical protrusion 20 may include one or more gaps / slots 21 formed to help indicate alignment with indicia 25 provided on the barrel 2. A cap 23 is attached, for example using adhesive, to the end of the guide element 13 opposite to the annular lip 16. The cap 23 is sized and preferably textured / formed of material to allow a user to readily grip and twist the guide element 13. The guide element 13 is received into the aperture 19 and screwed in (by turning the cap 23) until the end of the guide element 13 (at least the protruding ball bearing 14) is received within the helical track 6. The guide element 13 is secured by the thread 18 to prevent the guide element 13 moving back out of the helical track 6. If the cap 23 is then turned further to, the annular lip 16 will be forced against the barrel 2, preventing relative movement of the sleeve 5 and barrel 2 by friction. In this way, the cap 23, guide element 13 and cylindrical protrusion 20 also provide a locking mechanism which is actuatable to secure the relative position of the sleeve 5 relative to the barrel 2. In the first metered dose syringe 1, the protrusion of the ball bearing 14 beyond the annular lip 16 interacts with pre-set positions defined by recesses 24 in the helical track 6, described in further detail hereinafter. Recesses 24 may also provide additional mechanical security when locking the relative movement of barrel 2 and sleeve 5. In the first metered dose syringe 1, a pitch of the helical coupling varies along a range of motion of the helical coupling. This is most readily observed in the exploded views of Figures 4A to 4C. This is achieved by varying the pitch of the helical track 6 along its length. In other words, as the sleeve 5 is rotated relative to the barrel 2, the corresponding rate of axial translation changes as a function of the position of the sleeve 5 relative to the barrel 2 (within the range permitted by movement of the guide element 13 within the helical track 6). In the general case, the variable pitch of the helical coupling may be described as: dz — = (1) In which z is the axial displacement of the sleeve 5 relative to the barrel 2 parallel to the axis of the barrel 2, 8 is an angle of rotation of sleeve 5 relative to the barrel 2 about the axis of the barrel 2, and f(z) is a function parameterised on axial displacement z in Equation (1), but which could equally be parameterised in terms of rotational angle 8 or distance along the helical track 6 itself (since all three are linked and may to an extent be considered interchangeably). The axial displacement z and rotational angle 6 may be measured with either end of the range of motion as reference point (zero), which end is not important for this discussion. A fixed-pitch helical track 6 would correspond to f(z) being simply a scalar constant. Therefore, the helical coupling has variable pitch whenever the function f(z) is not constant with axial displacement z. The function f(z) may be continuous, for example proportional to z, or discontinuous such as transitioning through two or more regions of different constant values. The function f(z) may be expressible as a closed form expression, but this is not required. In the first metered dose syringe 1 as illustrated, the function f(z) takes the form of three constant values, separated by a pair of steplike transitions. As a result of the variable pitch of the helical track 6, the sensitivity of rotating the sleeve 5 of the first metered dose syringe 1 to set a maximum dischargeable volume (and hence a dose for a given medicament) may be varied across the range of motion of the sleeve 5 relative to the barrel 2. This permits setting accurate doses for patients spanning a broader range of weights. This may be particularly useful for use in paediatric settings, allowing a low pitch f(zi) at one end of the helical coupling range of motion for fine control of doses for newborns / infants, and transitioning (whether gradually or a step-change) to a larger pitch f(z2) >f(zi) as the sleeve 5 continues to be rotated about the barrel 2, so that doses may also be selected for older / larger infants, toddlers, and so forth. One or more indicia 25 may preferably be provided on the barrel 2 and / or sleeve 5, the one or more indicia indicating a dose, volume, patient age, patient weight etc associated with the relative positions of the sleeve 5 and barrel 2. Referring in particular to Figure 3A, in the first metered dose syringe 1, indicia 25 are provided on the barrel 2, whilst the sleeve 5 includes a window 26 arranged to provide visibility of the indicia 25 provided on the barrel 2. The window 26 is sized to allow only a single indicia 25 provided on the barrel 2 to be visible at once. This may help to prevent confusion between adjacent and / or closely spaced indicia 25. If present, the gap / slot 21 may further help to indicate alignment with the indicia 25 visible in the window 26. In other examples, the end of the sleeve 5 closest to the barrel 2 outlet 7, or a reference indicia (not shown) provided at a point on the circumference thereof, may additionally or alternatively be used in cooperation with Indicia 25 provided on the barrel 2 to indicate the volume / dose set. The indicia 25 may indicate a volume which is dispensable by depressing the plunger 4 until it abuts the sleeve 5. More preferably the metered dose syringe 1 will be prefilled with a particular medicament at a particular concentration, in which case the indicia 25 may preferably indicate a dose of the pre-filled medicament dispensable by depressing the plunger 4. Alternatively, when the metered dose syringe 1 is for use with a particular medicament at a particular concentration, the indicia 25 may instead indicate a weight or a weight range for whom the corresponding volume represents the correct dose. In another option still, when the metered dose syringe 1 is for use in paediatric medicine with a particular medicament at a particular concentration, the indicia 25 may indicate an age or age range of the patient, for example corresponding to a dose suitable for the average or ideal weight of a patient having the indicated age. Without wishing to be bound by speculation, the inventors consider that indicia 25 corresponding to patient weight may represent the most useful option in practice. The first metered dose syringe 1 should preferably also include space on the sleeve 5 for a label 27 or other indicia clearly Indicating the contents and concentration of a pre-filled syringe. Referring in particular to Figures 4A through 4D, the helical coupling may be configured to generate an audible and / or haptic indication in response to a position of the sleeve 5 relative to the barrel 2 corresponding to one of a number of pre-set relative positions defined by recesses 24. This is implemented in the metered dose syringe 1 by recesses 24 formed into the helical track 6. Each recess 24 has increased depth (in a direction radial in relation to the barrel 2 axis) compared to intervening sections of the floor 28 of the helical track 6. As the guide element 13 comes into coincidence with a recess 24, the force from spring 17 pushes the ball bearing 14 into the recess, providing haptic feedback to a user. The transition between each recess 24 and intervening sections of helical track 6 floor 28 may be smooth or sharp, and a sharp transition such as a step edge may also cause an audible "click" as the ball bearing 14 snaps into the recess 24. Each pre-set position defined be a recess 24 may correspond to one of the indicia 25. When the first metered dose syringe 1 is for use with a particular medicament at a particular concentration, each recess 24 will correspond to a particular dose. For example, each recess 24 may correspond to a dose corresponding to a patient age / welght / weight range as described hereinbefore in relation to indicia 25. In addition to increased depth, the walls of the helical track 6 also widen to provide a widened region 29 centred at each recess 24. The widened region 29 corresponds to a diameter equal to the annular lip 16, such that tightening the guide element 13 by twisting the cap 23 at that position will move the annular lip 16 into the widened region, below a wall height of the helical track 6. In this way, the relative movement of sleeve 5 and barrel 2 may be additional secured (or locked) to the pre-set position corresponding to the recess 24. This prevents any inadvertent movement, and hence dose variation, whilst depressing the plunger 4. Second metered dose syringe Referring also to Figures 5A and 5B, a second example of a metered dose syringe 30 (hereinafter the "second metered dose syringe") is shown. Figure 5A is a schematic projection view, and Figure 5B shows an expanded view of the region labelled "H" in Figure 5A. Referring also to Figures 6A to 6E, Figure 6A shows the second metered dose syringe 30 in a schematic side view. Figure 6B shows a cross-section along the plane denoted by label "K" in Figure 6A, and Figure 6C shows a cut-away projection from the same plane. Figure 6D shows a cross-section along the plane denoted by label "J" in Figure 6A. Figure 6E shows an expanded view of the region labelled "M" in Figure 6C. Some details, such as the specific structures of the plunger protrusion 9 and head 8 are not shown to the same level of detail in Figure 5A to 6E, but are nonetheless the same as for the first metered dose syringe 1. With the exception of the structural differences explicitly described hereinafter, the second metered dose syringe 30 Is the same as the first metered dose syringe 1. The first metered dose syringe 1 defines the helical coupling using the helical track 6 formed in the barrel 2 and cooperating with the guide element 13 coupled to the sleeve 5. By contrast, in the second metered dose syringe 30 the helical coupling is defined by a helical track 31 formed in the sleeve 5 in the form of a slot, cooperating with a second guide element 32 (or "second element") In the form of a protrusion integrally formed with (or otherwise securely connected to) the barrel 2. The second metered dose syringe 30 as illustrated relies on friction between the guide element 32 and the walls of the helical track 31 to prevent unwanted relative motions between the sleeve 5 and barrel 2. However, a locking mechanism may be added in a relatively straightforward manner. For example, the second guide element 32 in the form of a protrusion could be replaced with a cylindrical protrusion with an external thread (not shown). A cap (not shown) having an internally threaded hole (not shown - similar to a nut) could be screwed onto such a cylindrical protrusion, and tightened against the outside of the sleeve 5 to provide a locking mechanism. Additionally, recesses (not shown) having a diameter corresponding to the cap (not shown) could be formed at pre-set positions on the exterior surface of the sleeve, allowing the cap to be screwed into said recesses (not shown) to provide further mechanical security. Experimental tests Experimental tests were conducted using a test metered dose syringe configured In accordance with the first metered dose syringe 1. Referring also to Figures 7A and 7B, photographs of the test metered dose syringe used to obtain experimental results are presented. The test metered dose syringe was prepared to test the helical coupling mechanism, and as such was not used to contain liquid or dispense liquid to patients. The experiments were performed with 17 doctors, including a mixture of junior anaesthetists, consultants, and paediatricians), in one-on-one sessions. The participants Included 8 junior anaesthetics trainees, 4 senior anaesthetics trainees, 1 consultant anaesthetist, 2 paediatrics senior trainees, 1 consultant in intensive care / emergency medicine and 1 consultant in intensive care / emergency medicine / prehospital emergency medicine. The briefing provided to participants was: "This 4 year old child is in cardiac arrest. CPR is ongoing, and the rhythm has been confirmed as PEA. IV access is in situ. Please prepare and administer a dose of adrenaline". Participants were asked to use the equipment provided, which initially was a conventional adrenaline bolus. The scenario was then changed to 5 year old child, and the participant was given the test metered dose syringe to complete the task. The test metered dose syringe was initially supplied at the 'zero dose' condition, i.e. the sleeve 5 was fully extended with relation to the barrel 2, such that the plunger 4 has no ability to be depressed. This represents the condition in which an actual pre-filled syringe would be supplied (for safety reasons to avoid accidental overdoses). Referring also to Figure 8, the data for the 17 participants and the two tasks are presented. The hatched columns correspond to the time to complete the task using the conventional adrenaline bolus, and the solid black columns correspond to the time to complete the task using the test metered dose syringe. Note that no hatched column is indicated for participant "B" because they indicated that they did not feel confident to complete the task in a paediatric context using the conventional adrenaline bolus. It may be observed that using the test metered dose syringe, the time taken to complete these time-pressured tasks was significantly reduced, from a mean of 191 seconds to a mean of 41 seconds (to the nearest second). Additionally, the variability between participants was also reduced, with standard deviation for the conventional adrenaline bolus representing 53% of the respective mean, whereas this was reduced to 39% for the test metered dose syringe. Referring also to Table 1, the dosing errors determined from post-task reviews are presented: adrenaline bolus test metered dose syringe Correct dose 11 17 Serious dosing error 4 0 Near miss 2 0 Table 1 Of the 4 serious dosing errors using the conventional adrenaline bolus, one participant did not complete the task, two participants prepared a dose which was an order of magnitude too low (ineffective dose), and one participant prepared a dose which was 10 times too large (overdose). The 2 "near misses" corresponded to one participant who made an order of magnitude error in their calculations but self-corrected during the task, and another participant drew up 10 times the correct dose before self-correcting. Whilst further and more extensive clinical testing is needed, these initial results are already sufficient to clearly demonstrate that metered dose syringes according to the present disclosure are, compared to a conventional bolus prepared at the time, significantly faster, more consistent (reduced variability between participants), and also safer against errors in preparing the correct dose. Further examples The first 1 and second 30 metered dose syringes have been illustrated with particular structures and using particular mechanisms to aid understanding of the central concepts of metered dose syringes according to the present disclosure and using a helical coupling. However, metered dose syringes according to the present disclosure may be implemented using different features or a subset of features compared to the first 1 and second 30 metered dose syringes, with the essential elements being that the sleeve 5 is helically coupled to the barrel 2, and that the sleeve is configured to restrict travel of the plunger when depressed, such that rotating the sleeve relative to the barrel controls a maximum delivery volume from depressing the plunger. For example, whilst the helical coupling is provided by helical track 6 in the first metered dose syringe 1 and by helical track (slot) 31 in the second metered dose syringe 30, this is not essential. In other examples, the helical coupling could be provided by, for example, one or more ball bearings (not shown) disposed within a ball-bearing run partly defined by the barrel 2 and partly defined by the sleeve 5 (for example, corresponding helical grooves). When a helical track 6, 31 is used, a corresponding guide element 13, 32 may in general be either integrally formed with, or else coupled to, whichever of the barrel 2 and sleeve 5 does not define the helical track 6, 31. In general, a guide element 13, 32 for cooperating with a helical track 6, 31 may take a variety of forms other than those described in relation to the first 1 and second 30 metered dose syringes, provided that the guide element is not a thread. For example, a guide element 13, 32 may take the form of an extension or protrusion projecting radially inwards from the sleeve (as guide element 13), or radially outwards from the barrel 2 (as second guide element 32). When coupled to one of the barrel 2 or sleeve 5, a guide element 13, 32 may be statically coupled or fixed, or instead may be coupled to the sleeve 5 or barrel 2 via one or more bearings arranged to permit movement of the guide element 13, 32, or a portion of the guide element 13, 32 which contacts the helical track (for example ball bearing 14), radially relative to the sleeve 5 or barrel 2 (as the case may be). When used, such bearings preferably minimise or constrain axial and / or rotational movements of the guide element 13, 32 relative to the sleeve 5 or barrel (as appropriate). In other words, the guide element 13, 32 may be constrained to move together axially and rotationally with whichever of the sleeve 5 or barrel 2 it is coupled to, relative to the other of the barrel 2 and sleeve 5. The guide element 13 in the form of a threaded ball and spring plunger, as illustrated in relation to the first metered dose syringe 1, is one example of such bearings. In general, the one or more bearings may include, without being limited to, one or more of plain bearings (not shown), ball bearings 14, springs 17 (or other types), flexures 33 (Figure 9A), flexures 33 (Figure 9A) integrally formed with the sleeve 5 or barrel 2, or any other bearing type(s) known to be suitable to provide the relative motions and constraints described hereinbefore. In general, the guide element 13, 32, or a portion thereof, which is received into the helical track 6 may take a variety of shapes. In addition to spherical / hemispherical (as for the ball bearing 14 of the first metered dose syringe 1), a portion of the guide element 13, 32 which is received into the helical track 6 may be cylindrical, pyramidal, conical, or may have any shape complementary to a profile of the helical track 6. The respective helical tracks 6, 31 of the first 1 and second 30 metered dose syringes are straight-walled in cross-section. Alternatively, one or both walls defining a helical track 6, 31 may be tapered in cross-section. The walls defining the respective helical tracks 6, 31 of the first 1 and second 30 metered dose syringes have a profile along the length of the helical track 6, 31 which is a smooth curve. Alternatively, the walls defining a helical track may have a profile along the length of the track which is sawtoothed, geared, ratcheted, stepped, and so forth. In this way, pre-set positions may be defined using the walls of the helical track 6, 31, rather than a recess 24 in the floor 28 thereof. This could be applied to, for example, the slot-like second helical track 31, which does not include a floor 28. Another option for replacing recesses 24 in the floor 28 of a helical track 6 to delimit pre-set positions is to instead provide corresponding protrusions 39 (Figures 9B) extending up into the helical track 6 from the floor 28 (or base) of the helical track 6. Instead of the floor 28, such protrusions 39 (Figure 9B) may instead extend into the helical track 6 from one or both walls defining the track 6. When a recess 24 or protrusion 39 (Figure 9B) is used to define pre-set positions, transitions between each recess 24 / protrusion 39 (Figure 9B) and intervening sections of the respective helical track 6, 31 may be smooth on one side (i.e. in one direction) and sharp on the other side (i.e. in the opposite direction). This may facilitate latching between the guide element 13, 32 and the recess 24 / protrusion, as part of a locking mechanism of a metered dose syringe 1, 30. In the first metered dose syringe 1, the ball bearing 14 of the guide element 13 is urged towards the helical track 6 by biasing means in the form of spring 17. However, other biasing means may provide the same or similar functionality such as, for example, a flexure, an elastomeric member, and so forth. In this way, the guide element 13, 32 will be pushed into recesses 24 defining pre-set positions. Further movement along the helical coupling will require forcing the guide element 13 (or portion thereof such as ball bearing 14) out of the recess against the action of the biasing means (such as the spring 17), which will provide haptic feedback to a user that a pre-set position has been reached. In some examples, the combination of guide element 13, 32 and recesses 24 / protrusions 39 (Figure 9B) may be configured to form a series of detents. In other words, the sleeve 5 and barrel 2 may "lock in" to the corresponding pre-set position until released by force exceeding a threshold is applied, a release catch is actuated, and so forth. In general, a detent may be provided if, once a guide element 13, 32 is received within a recess 24 (or a protrusion 39 is received within the guide element 13, 32), force applied to rotate the sleeve 5 relative to the barrel 2 does not generate a component of reaction force oriented to cause the guide element 13, 32 to be removed from the recess 24 (overcoming biasing means such as spring 17 if included). Detents are not limited to recesses 24, and depending on the relative shapes of guide element 13 and a protrusion (Figure 9B), that protrusion may also provide a detent. If the edge of a detent / recess 24 / protrusion 39 (Figure 9B) is configured with a sharp transition, an appropriately shaped guide element 13, 32 (for example sharp on a trailing side) may generate an audible "click" as the guide element 13, 32 passes the sharp transition. Alternative mechanisms for generating an audible and / or haptic "click" may be employed. When a locking mechanism is included to help additionally secure the relative positions of the sleeve 5 and barrel 2, this may take the form of a screw, as with the threaded outer surface 18 of guide element 13 of the first metered dose syringe 1. Other forms of locking mechanism based on a screw may be used, for example using any configuration arranged so that the screw may be tightened to increase a force biasing the guide element 13, 32 towards / into the helical track 6, 31. Alternatively, a locking mechanism may include, or take the form of, a clutch, a wedge, a spline, a spring ratchet mechanism - for example cooperating with one or more recesses / protrusions / detents and so forth. Further examples of suitable locking mechanisms may include, or take the form of, an irreversible snap-fit mechanism, a friction fit mechanism, a cantilever arm and so forth. In other words, a locking mechanism may take any form previously employed in the field of mechanics for the purposes of resisting and / or preventing relative motion between a pair of parts. In the first metered dose syringe 1, the locking mechanism provided by turning the cap 23 to tighten the guide element 13, 32 into the helical track 6 is actuatable at any point within the range of motion of the helical coupling (i.e. at any point of the helical track 6). This functionality may be generally applied to other metered dose syringes in accordance with the present specification. A locking mechanism may be additionally secure when actuated at a position corresponding to a pre-set position, for example locations corresponding to recesses 24 as described hereinbefore in relation to the first metered dose syringe 1. Alternatively the locking mechanism may only be actuatable at a pre-set positions (not shown). Locking mechanisms have been described which are positively actuated to secure the relative positions of the sleeve 5 and barrel 2. Alternatively, if the helical coupling is provided with recesses 24 or protrusions cooperating with respective guide elements 13, 32 to form detents, the sleeve 5 and barrel 2 will automatically lock into each preset position as they are rotated. The locking mechanism may then take the form of comprising a release mechanism actuatable to release the locking mechanism and permit further relative movement of the sleeve 5 and barrel 2. In the first 1 and second 30 metered dose syringes, the outlet 7 has been shown as a Luer taper. However, in general metered dose syringes 1, 30 according to the present specification may be configured with any desired type of outlet 7, including but not limited to a Luer slip, a Luer lock, a threaded connection, a snap-fit connection, an integral needle, or any other standard fitting such as, for example, an NRFit (RTM) connection. When supplied, a removeable cap (not shown) may be positioned to cover the outlet 7 of the barrel 2. The removeable cap (not shown) may be single use such that once first removed, the removable cap may not be replaced. This may provide visual evidence that a metered dose syringe 1, 30 has been damaged or tampered with, and may help to prevent improper re-use of the metered dose syringe 1, 30. The alternative mechanisms, features and functions described hereinbefore may be combined in any compatible manner. It shall be helpful to illustrate several illustrative examples for providing haptic feedback and / or locking mechanisms suitable for metered dose syringes 1, 30 in accordance with the present specification. First example configuration Referring also to Figure 9A, a first example configuration 34 is shown. Figure 9A is a schematic cross-section, taken along the path of a helical track 6 formed in the barrel 2. In the first example configuration 34, pre-set positions are provided by recesses 35 into the floor 28 of the helical track 6 formed in the barrel 2. Each recess 35 has a surface corresponding to a portion of a sphere (a portion less than a hemisphere). A flexure 33 is connected to, or integrally formed as part of, the sleeve 5. An end of the flexure 33 which is not connected to the sleeve 5 terminates in a hemispherical guide element 36. The flexure 33 is formed such that, when the barrel 2 is received within the sleeve 5 with the hemispherical guide element 36 received in the helical track 6, the flexure 33 is pre-stressed to provide a force urging the hemispherical guide element 36 towards the barrel 2 (i.e. into the helical track 6). As the sleeve 5 is rotated relative to the barrel 2, when the hemispherical guide element 36 reaches a recess 35, it is urged into the recess 35 by the stressed flexure 33. The shapes of the recess 35 and hemispherical guide element 36 mean that the sleeve 5 may continue to be rotated provided enough force is applied to move the hemispherical guide element 36 against the force of the flexure 33. This will provide haptic feedback to a user as they rotate the sleeve 5 and barrel 2 past each pre-set position. A locking mechanism is provided in the form of a screw 37 which may be tightened against the flexure 33 corresponding to the location of the hemispherical guide element 36 so as to prevent the hemispherical guide element 36 being withdrawn from a recess 35. In this was, the relative positions of sleeve 5 and barrel 2 may be locked to the pre-set position corresponding to the recess 35 receiving the hemispherical guide element 36. Whilst the first example configuration 34 has been illustrated with reference to a helical track 6 formed into the barrel 2, the same concepts are equally applicable to examples In which the helical track 6 is instead formed Into the sleeve 5. Second example configuration Referring also to Figure 9B, a second example configuration 38 is shown. Figure 9B is a schematic cross-section, taken along the path of a helical track 6 formed in the barrel 2. The second example configuration 38 is the same as the first example configuration 34, except that instead of recesses 35, protrusions 39 are formed extending upwards from the floor 28 of the helical track 6. Each protrusion 39 has a surface corresponding to a portion of a sphere (a portion less than a hemisphere). In order to receive the protrusions 39 as it moves along the helical track 6, the guide element 13, 32 takes the form of a thickened portion 40 at the end of the flexure 33 which is not connected to (or integrally formed with) the sleeve 5, and a recess 41 is formed in the thickened portion 40 as the inverse of one of the protrusions 39. The edges of the thickened portion 40 which contact the floor 28 are slightly curved so as to facilitate movement of the thickened portion 40 up and over the protrusions 39 until they are received Into the recess 41. Whilst the second example configuration 38 has been illustrated with reference to a helical track 6 formed into the barrel 2, the same concepts are equally applicable to examples in which the helical track 6 is instead formed into the sleeve 5. Whilst the first 34 and second 38 example configurations have been illustrated with shapes of recesses 35 / protrusions 39 and guide elements 13, 32 which may be moved (unless locked using screw 17) at the cost of additional force to deform the flexure 33, in other examples the recesses 35 / protrusions 39 and guide elements 13, 32 may instead be shaped so that the guide elements 13, 32 would lock into each pre-set position until released to permit continued movement by a user actuatable mechanism (see for example shapes shown in Figures 10A and 10B). Third example configuration Referring also to Figure 10A, a third example configuration 42 is shown. Figure 10A Is a schematic cross-section, taken along the path of a helical track 6 formed in the barrel 2. The third example configuration 42 differs from the first example configuration 34 in several ways. Firstly, the recesses 43 of the third example configuration 42 have a step-like profile along the length of the helical track 6, with edges substantially perpendicular to the floor 28. Outside the cross section shown in Figure 10A (which is not a plane because it follows the path of the helical track 6), recesses 43 may be square, circular, rectangular, or any other shape. The guide element 13, 32 has a corresponding shape to the recesses 43, and is dimensioned to just fit into the recesses 43. A shaft 44 extends radially (relative to the barrel 2) away from the back of the guide element) and is received by a through hole In the sleeve 5 (or another element coupled to the sleeve 5). The guide element 13, 32 is biased towards the floor 28 by a helical spring 45 received over the shaft 45 and between the guide element 13, 32 and an interior surface of the sleeve 5. The helical spring is preferably pre-stressed, i.e. there is not enough range of motion of the shaft 44 to allow the helical spring 45 to reach its natural length. As the sleeve 5 is rotated relative to the barrel 2, the guide element 13, 32 slides along the floor 28 of the helical track 6 until a recess 43 is encountered and the guide element 13, 32 is pushed into the recess by the helical spring 45. Unlike the first 34 or second 38 example configurations, further force directed along the length of the helical track 6 does not generated any reaction force component between the guide element 13, 32 and the walls of the recess 43 which is directed upwards (i.e. out of the recess 43). Therefore, the sleeve 5 will remain locked in position relative to the barrel 2 unless and until the shaft 44 is actuated to withdraw the guide element 13, 32 from the recess 43, for example by a user actuatable button, lever etc. The sides of the recesses 43 need not be step-like. Curved profiles, even portions of spherical surfaces may be used provided that any component of reaction force remains, in practical circumstances of twisting force applied by a human, insufficient to overcome the biasing of the helical spring 45. Whilst the third example configuration 42 has been illustrated with reference to a helical track 6 formed into the barrel 2, the same concepts are equally applicable to examples in which the helical track 6 is instead formed into the sleeve 5. For example, a ball and spring plunger (not shown) may be used to push a guide element 32 in the form of a ball bearing (not shown) radially outwards from the barrel 2 and into a helical track 31 formed in the sleeve 5. Pre-set positions may be determined by placement of through-holes (not shown) along the helical track 31. The size of the through-holes (not shown) may be configured such that when the ball bearing (not shown) is received by a through-hole, further rotation of the sleeve 5 and barrel 2 is not possible until a user presses the ball bearing (not shown) back into the through-hole to release the helical coupling. Fourth example configuration Referring also to Figure 10B, a fourth example configuration 46 is shown. Figure 10B is a schematic cross-section, taken along the path of a helical track 6 formed In the barrel 2. The fourth example configuration 46 is the same as the third example configuration 42, except that the recesses 43 are replaced by protrusions 47 and the guide element 13, 32 has a recess 48 formed therein to receive a protrusions 47. A leading edge 49 of the guide element 13, 32 is curved, or otherwise shaped, to facilitate the guide element 13, 32 moving up and over a protrusion 47 until recess 48 receives the protrusion 47. Optionally the leading and / or trailing edges of protrusion 47 may also be curved, bevelled and so forth to facilitate easy movement of the guide element 13, 32 up until it is locked be receiving a protrusion 47 into recess 48. Whilst the fourth example configuration 46 has been illustrated with reference to a helical track 6 formed into the barrel 2, the same concepts are equally applicable to examples in which the helical track 6 is instead formed into the sleeve 5. Pre-filled syringes A pre-filled syringe (not shown) is made by filing the internal volume 3 within the barrel 2 of metered dose syringe 1, 30 according to the present specification with a liquid medicament (not shown). The medicament may include one or more active ingredients dissolved and / or suspended in the liquid. For example, adrenaline is typically provided as a solution, whereas 2,6-diisopropylphenol (propofol) is typically provided as an emulsion. A particularly important used case for a pre-filled syringe is one filled with a medicament in the form adrenaline at a dilution of 1:10,000, which may alternatively be expressed as 100 micrograms per millilitre. In a case where the pre-filled syringe is intended for paediatric use, a dose of adrenaline may be set to a value corresponding to 10 micrograms of adrenaline per kilogram of the patient weight. Pre-set positions of a pre-filed syringe may be set (for example using recesses 24, 35, 43 or protrusions 39, 47) to correspond to dose of 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 micrograms of adrenaline (spanning a weight range from 1 to 10 kg). By way of background, and without wishing to be bound to the following doses, the current dosing regimen for paediatric emergencies in the United Kingdom (UK) is based on the Resuscitation Council Paediatric Emergency Drug Chart (https: / / www.resus.org.uk / sites / default / files / 2021-05 / 2492o / o20AAP%20RCUK%20PET%20chart-5.pdf). This chart is reproduced for reference in Figure 11, and provides details of other medicaments commonly used during emergencies, including glucose and sodium bicarbonate. These other medicaments may also be used to fill the internal volume 3 of metered dose syringes 1, 30 in accordance with the present specification. For a child aged <1 month, the estimated weight is 3.5 kg, corresponding to volume of adrenaline at 1:10,000 dilution of 0.35 mL (35 micrograms). For a child 1 month old, estimated weight is 4 kg, and volume of adrenaline at 1:10,000 dilution is 0.4 mL (40 micrograms). For a child 3 months old, estimated weight is 5 kg, and volume of adrenaline at 1:10,000 dilution is 0.5 mL (50 micrograms). For a child 6 months old, estimated weight is 7 kg, and volume of adrenaline at 1:10,000 dilution is 0.7 mL (70 micrograms). For a child 1 year old, estimated weight is 10 kg, and volume of adrenaline at 1:10,000 dilution is 1.0 mL (100 micrograms). For a child 2 years old, estimated weight is 12 kg, and volume of adrenaline at 1:10,000 dilution is 1.2 mL (120 micrograms). For a child 3 years old, estimated weight is 14 kg, and volume of adrenaline at 1:10,000 dilution is 1.4 mL (140 micrograms). For a child 4 years old, estimated weight is 16 kg, and volume of adrenaline at 1:10,000 dilution is 1.6 mL (160 micrograms). For a child 5 years old, estimated weight is 18 kg, and volume of adrenaline at 1:10,000 dilution is 1.8 mL (180 micrograms). For a child 6 years old, estimated weight is 20 kg, volume of adrenaline at 1:10,000 dilution is 2.0 mL (200 micrograms). For a child 7 years old, estimated weight is 23 kg, and volume of adrenaline at 1:10,000 dilution is 2.3 mL (230 micrograms). For a child 8 years old, estimated weight is 26 kg, volume of adrenaline at 1:10,000 dilution is 2.6 mL (260 micrograms). For a child 10 years old, estimated weight is 30 kg, and volume of adrenaline at 1:10,000 dilution is 3 mL (300 micrograms). For a child 12 years old, estimated weight is 38 kg, and volume of adrenaline at 1:10,000 dilution is 3.8 mL (380 micrograms). For a child 14 years old or adolescent, estimated weight is 50 kg, volume of adrenaline at 1:10,000 dilution is 5 mL (500 micrograms). For an adult, estimated weight is 70 kg, and volume of adrenaline at 1:10,000 dilution is 10 mL (1 mg). As can be seen from Figure 11 and the values outlined hereinbefore, the range of "paediatric emergency" spans patients ranging in weight from about 3.5 kg up to about 50 kg, with corresponding does of adrenaline spanning 35 micrograms to 500 micrograms. The use of a helical coupling with variable pitch is particularly advantageous in this application, as it permits spanning a wide range, whilst ensuring the accuracy at each point of the helical coupling is suitable. Using metered dose syringes 1, 30 according to the present specification, in particular with helical couplings of variable pitch, a single pre-filled syringe containing adrenaline may be prepared having pre-set positions corresponding to doses of adrenaline: • Less than or equal to 35 micrograms; • More than 35 micrograms and less than or equal to 45 micrograms; • More than 45 micrograms and less than or equal to 60 micrograms; • More than 60 micrograms and less than or equal to 85 micrograms; • More than 85 micrograms and less than or equal to 110 micrograms; • More than 110 micrograms and less than or equal to 130 micrograms; • More than 130 micrograms and less than or equal to 150 micrograms; • More than 150 micrograms and less than or equal to 170 micrograms; • More than 170 micrograms and less than or equal to 190 micrograms; • More than 190 micrograms and less than or equal to 215 micrograms; • More than 215 micrograms and less than or equal to 245 micrograms; • More than 245 micrograms and less than or equal to 280 micrograms; • More than 280 micrograms and less than or equal to 340 micrograms; • More than 340 micrograms and less than or equal to 440 micrograms; • More than 440 micrograms and less than or equal to 750 micrograms; and • More than 750 micrograms. In other examples, the medicament used to fill a metered dose syringe 1, 30 according to the present specification may include, without being limited to, one or more of amiodarone, calcium chloride, atropine, or any other substance known for use in treating or managing cardiac arrest and producible in liquid form (whether dissolved or in suspension). In further examples, the medicament used to fill a metered dose syringe 1, 30 according to the present specification may include, without being limited to, one or more of glucose 10%, lorazepam, adenosine, or any other substance known for use in treating or managing medical emergencies / critical injuries and producible in liquid form (whether dissolved or in suspension). In still further examples, the medicament used to fill a metered dose syringe 1, 30 according to the present specification may include, without being limited to, one or more of propofol, ketamine, rocuronium, fentanyl, alfentanil, midazolam, morphine, metaraminol, ephedrine, phenylephrine, or any other substance known for use in Anaesthesia and / or Rapid Sequence Induction. Use method and priming Syringes need to be "primed" before use to remove any trapped gas bubbles and ensure that these are not injected into a patient. A priming step is possible using a pre-filled syringe made by filling the internal volume 3 of a metered dose syringe 1, 30 according to the present specification with medicament. In particular, the relative position of the sleeve 5 and the barrel 2 is initially set to a start position corresponding to an extreme end of the helical coupling range of motion which minimises the travel of the plunger 4 when depressed. In other words, the initial position of the sleeve 5 and the barrel 2 so that a far end of thew sleeve 5 is as far as possible from the outlet 7. Then the internal volume 3 is filled with medicament such that In its initial position the plunger 4 may be depressed to discharge a predetermined priming volume before abutting the sleeve 5 in its initial position. In this way, the pre-filled syringe may be used by first removing it from sterile packaging (necessary for hygiene and infection control), then priming the pre-filled syringe to ensure any gas is removed by depressing the plunger until it abuts the sleeve 5 in its initial position. During this priming step, the pre-filled syringe should be "inverted", which is to say held with the outlet 7 of the barrel 2 uppermost. This will also help to ensure a reliable zero-level before setting the dispensable volume. Once the pre-filled syringe is primed, the sleeve 5 and the barrel 2 are rotated (with axial translation generated by the helical coupling) to set the required volume / dose. If included, the locking mechanism may be engaged (manually or automatically as described herein). The pre-filled syringe is then ready to be used to dispense the set volume / dose by depressing the plunger 4. This may be directly into a patient if the outlet 7 takes the form of a needle, but may more often be into a pre-existing port or line of an intravenous / intraosseous access device, or possibly to an intramuscular / subcutaneous / subdermal needle. In addition to being packaged in sterile packaging, the pre-filled syringe may also include a cap or cover (not shown) which must be removed from the barrel 2 outlet 7 prior to use. This may also help to avoid any inadvertent discharging of the priming volume before removal of the cap. The cap or cover may be irreversibly removable, which may facilitate detection of tampering. Modifications It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, configuration and use of metered dose syringes and similar devices for dispensing controlled quantities of medicaments, and which may be used instead of or In addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment. Metered dose syringes according to the present specification, for example the first 1 and / or second 30 metered dose syringe, may optionally include a capture mechanism (not shown) configured to secure the plunger 4 to the sleeve 5 once the plunger 4 has been depressed to deliver the maximum delivery volume set using the sleeve 5. The capture mechanism should not be operable until the after the plunger 4 has been depressed to abut the sleeve 5 for a first time to prime the metered dose syringe 1, 30. The capture mechanism (not shown) may take the form of a catch or similar mechanism configured to lock the plunger 4 to the sleeve 5 and prevent withdrawal of the plunger 4. The sleeve 5 already prevents further depression of the plunger 4. The capture mechanism (not shown) may be further configured to prevent relative rotation of the sleeve 5 relative to the barrel 2 once the plunger 4 has been secured to the sleeve, so that medicament remaining in the internal volume 3 cannot be discharged by rotating the now locked plunger 4 and sleeve 5 relative to the barrel 2. In this way, after being first used as intended, a metered dose syringe according to the present specification cannot be further used and / or re-used. For example, a one-directional ratchet mechanism may be coupled between the plunger 4 and the barrel 2, or between the plunger 4 and the sleeve 5. Alternatively, the plunger 4 and the sleeve 5 may be configured such that, when the plunger 4 abuts the sleeve 5 to prevent further depression, a top of the plunger 4 will be surrounded by, and flush with, a top surface of the sleeve 5. In this way, it may be impossible to draw the plunger out again without additional tools. The capture mechanism (not shown) may further include a visual indicator that the plunger has been secured to the sleeve. This may provide visual confirmation to a user that the used metered dose syringe has been rendered inoperable. The capture mechanism (not shown) may require user actuation to engage, but once engaged may be irreversible without physically breaking the capture mechanism. In some examples, the capture mechanism may engage automatically when the plunger 4 is depressed with sufficient force whilst abutting the sleeve 5. The force required to engage the capture mechanism may be configured in order to avoid accidental engagement when priming the metered dose syringe 1, 30. The capture mechanism may be further configured to provide an audible and / or haptic feedback when it is engaged, for example a "click". In this way, an authorised user can be certain both that they have delivered the entire dose they set by rotating the sleeve, and that they have made the syringe safe against misuse. Examples have been described in which indicia 25 are provided on the barrel 2 and / or the sleeve 5. In some further examples, indicia 25 may additionally or alternatively be provided on the shaft 10 of the plunger 4. For example, the plunger 4 may include gradations indicating volumes, doses etc. For example, the plunger 4 shaft 10 may include indicia dividing a range of motion into 10 separate 1 ml sections, with subdivisions of 0.1 ml or 0.2 ml. 5 Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or 10 not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom. 15

Claims

1. A metered dose syringe comprising:a barrel enclosing an internal volume for holding liquid;a plunger; anda sleeve received over and helically coupled to the barrel, wherein the helical coupling between the sleeve and barrel includes at most one threaded surface;wherein the sleeve is configured to restrict travel of the plunger when depressed, such that rotating the sleeve relative to the barrel controls a maximum delivery volume from depressing the plunger.

2. The metered dose syringe according to claim 1, wherein a pitch of the helical coupling varies along a range of motion of the helical coupling.

3. The metered dose syringe according to claims 1 or 2, wherein the helical coupling comprises a helical track and a guide element arranged to run along the helical track.

4. The metered dose syringe according to claim 3, wherein the helical track Is formed in the barrel, and wherein the guide element is a first element coupled to or integrally formed with the sleeve.

5. The metered dose syringe according to claim 3, wherein the helical track is formed in the sleeve, and wherein the guide element is a second element coupled to or integrally formed with the barrel.

6. The metered dose syringe according to any one of claims 1 to 5, wherein one or more indicia are provided on the barrel and / or sleeve, the one or more indicia indicating a dose or volume associated with the relative positions of the sleeve and barrel.

7. The metered dose syringe according to claim 6, wherein the sleeve comprises a window arranged to provide visibility of indicia provided on the barrel.

8. The metered dose syringe according to any one of claims 1 to 7, wherein the helical coupling is configured to generate an audible and / or haptic indication in response to a position of the sleeve relative to the barrel corresponding to one of a plurality of pre-set relative positions.

9. The metered dose syringe according to claim 8 when dependent via claim 3, wherein each of the plurality of pre-set relative positions are provided by corresponding recesses formed in the helical track.

10. The metered dose syringe according to any one of claims 1 to 9, further comprising a locking mechanism actuatable to secure the relative position of the sleeve relative to the barrel.

11. The metered dose syringe according to claims 8 or 9, further comprising a locking mechanism configured to engage at each of the plurality of pre-set relative positions, so as to secure the relative position of the sleeve relative to the barrel the locking mechanism comprising a release mechanism actuatable to release the locking mechanism and permit further relative movement of the sleeve and barrel.

12. The metered dose syringe according to any one of claims 1 to 11, wherein an outlet of the barrel is configured for coupling via a Luer taper, a Luer slip, a Luer lock, a threaded connection, or a snap-fit connection.

13. The metered dose syringe according to any one of claims 1 to 11, wherein an outlet of the barrel is coupled to an integral needle.

14. The metered dose syringe according to any one of claims 1 to 13, further comprising a capture mechanism configured to secure the plunger to the sleeve once the plunger has been depressed to deliver the maximum delivery volume set using the sleeve.

15. A pre-filled syringe comprising the metered dose syringe according to any one of claims 1 to 14, having the internal volume of the barrel filled with a liquid medicament.

16. The pre-filled syringe according to claim 15, wherein the medicament comprises adrenaline.

17. The pre-filled syringe according to claim 16, when depending via claims 2 and 8, comprising at least pre-set positions corresponding to doses of 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 micrograms of adrenaline.

18. The pre-filled syringe of any one of claims 15 to 17 wherein the relative position of the sleeve and barrel Is set to a start position corresponding to an extreme end of the helical coupling range of motion which minimises the travel of the plunger when depressed; andWherein the internal volume is filled such that the plunger may be depressed to discharge a pre-determined priming volume before abutting the sleeve in the start position.

19. A method of setting a metered dose syringe according to any one of claims 1 to 14 or a pre-filled syringe of any one of claims 15 to 18, comprising rotating the sleeve relative to the barrel to set a maximum delivery volume from depressing the plunger.

20. The method of claim 19, further comprising, before rotating the sleeve relative to the barrel to set the maximum delivery volume, priming the syringe by depressing the plunger to abut the sleeve In a starting position.

21. The method of claims 19 or 20, wherein the metered dose syringe or pre-filled syringe comprises a locking mechanism actuatable to secure the relative position of the sleeve relative to the barrel;the method further comprising, after rotating the sleeve relative to the barrel to set the maximum delivery volume from depressing the plunger, actuating the locking mechanism.

22. The method of claims 19 or 20, wherein the metered dose syringe or pre-filled syringe comprises a locking mechanism configured to engage at each of a plurality of pre-set relative positions of the helical coupling, so as to secure the relative position of the sleeve relative to the barrel, the locking mechanism metered dose syringe further comprising a release mechanism actuatable to release the locking mechanism and permit further relative movement of the sleeve and barrel;the method further comprising actuating the release mechanism one or more times whilst rotating the sleeve relative to the barrel to set the maximum delivery volume from depressing the plunger.

23. The method of any one of claims 19 to 22, wherein an outlet of the barrel of the metered dose syringe or pre-filled syringe comprises is configured for coupling via a Luer taper, a Luer slip, a Luer lock, a threaded connection, or a snap-fit connection;the method further comprising connecting the outlet of the barrel to an inlet via the respective Luer taper, Luer slip, Luer lock, threaded connection, or snap-fit connection.5 24. The method of any one of claims 19 to 23, wherein the metered dose syringeor pre-filled syringe comprises a capture mechanism configured to secure the plunger to the sleeve once the plunger has been depressed to deliver the maximum delivery volume set using the sleeve;wherein the method comprises pressing the plunger until the capture10 mechanism engages to secure the plunger to the sleeve.

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